1. Field of the Invention
The present invention relates to an apparatus for controlling the fuel injection quantity, in which an air/fuel (A/F) ratio feedback control and an A/F ratio open control can be switched.
2. Description of the Prior Art
Generally, a feedback control is performed in an internal combustion engine, in which a basic fuel injection quantity is computed on the basis of the intake air amount or the internal pressure of an intake pipe and the engine speed, and is corrected on the basis of the output of an O.sub.2 sensor installed in an exhaust pipe of the internal combustion engine. As a result, the fuel to be supplied into the engine can be adjusted to satisfy the desired A/F ratio such as the theoretical A/F ratio. For example, when the disturbance of the A/F ratio control such as the canister purge occurs, the coefficient (hereinafter, referred to as an .alpha. signal) which corrects the fuel injection quantity so as to obtain the theoretical A/F ratio, is computed in a control unit. The desired A/F ratio can be obtained by the A/F ratio feedback control in which the calculated coefficient value is used. Accordingly, when the canister purge is larger than the theoretical A/F ratio, the .alpha. signal is outputted in order to decrease the fuel injection quantity more than the case where the canister purge is not performed.
There is shown the prior countermeasure to the disturbance of the A/F ratio control in the laid-open patent application 2-70953 (1990), in which disclosed is such a technique that calculates the deviation from the standard value of the .alpha. signal, corrects it every operation regions so that the .alpha. signal may be stabilized at the standard value, and learns them. Therefore, even if the disturbance of the A/F ratio occurs, it is possible to control accurately the A/F ratio and supply stably the fuel injection, independent of the disturbance of A/F ratio control and the operation region, by detecting, correcting and learning the disturbance of A/F ratio in the control unit.
However, accurate control can be achieved only within the region of A/F ratio feedback control in which the oxygen sensor is activated. Therefore, during A/F ratio open control such as an A/F ratio lean burn control, the accuracy of A/F ratio control may be remarkably decreased due to the effect of A/F ratio control disturbance such as the canister purge.
Generally, the A/F ratio of the internal combustion engine is adjusted to the theoretical A/F ratio by the A/F ratio feedback control. Then, the condition for shifting to the A/F ratio open control is satisfied and the open control is performed. After shifting to the open control, the control must be performed independent of the value learned so far, because of the disturbance of canister purge. That is, the A/F ratio may be gradually varied, because the concentration of the gas vapor in the purge air from the canister, which is supplied to the internal combustion engine along with the intake air during the open control, may be changed over time. When the gas concentration of canister purge is decreased, the A/F ratio becomes lean, thus the driving performance may be deteriorated due to misfire, torque-down, and so on. On the other hand, when the gas concentration of canister purge is increased, the A/F ratio becomes rich, thus the components of exhaust gas may be deteriorated.
As described above, the learning correction amount in the A/F ratio feedback control can not be renewed in the A/F ratio open control. Therefore, there is such a problem that when the disturbance of the A/F ratio control is changed over time during the A/F ratio open control, the accurate correction can not made, the driving performance may be deteriorated, and the components of exhaust gas may also get worse.
Now, the gas concentration during the purge may be effected by various factors such as the temperature, the amount of gas and the gas characteristics. Therefore, it is very difficult to predict the gas concentration. As a result, when shifting to the A/F ratio open control, it is essential to recognize quickly and accurately and correct suitably the effect of the disturbance of A/F ratio. Furthermore, it is also essential to adjust the fuel injection quantity in accordance with the time varying amount of the disturbance of A/F ratio during the open control.